Separating xenon from krypton is an industrially important problem. Xenon (Xe) and krypton (Kr) are used in fluorescent light bulbs, and current technology produces these gases from the cryogenic distillation of air, in which these noble gases are present in small concentrations (1.14 ppmv Kr, 0.086 ppmv Xe). Both xenon and krypton separate into the oxygen-rich stream after distillation, and these gases are concentrated and purified to produce an 80/20 molar mixture of krypton to xenon.1 This final mixture typically undergoes further cryogenic distillation to produce pure krypton and pure xenon. Distillation is an energy-intensive process, and separation of these gases by selective adsorption near room temperature would be much more energy efficient. Additionally, separating krypton from xenon is an important step in removing radioactive krypton-85 during treatment of spent nuclear fuel.2 However, even after cryogenic distillation, trace levels of radioactive krypton in the xenon-rich phase are too high to permit further use.2 If adsorbents could reduce krypton-85 concentrations in the xenon-rich phase to permissible levels, there could be an entirely new supply source of xenon for industrial use. Thus, there is a strong need to develop adsorbent materials for this separation to reduce energy consumption and to reuse byproducts of consumed nuclear fuel.
There are several examples in the literature where zeolites have been tested for Xe/Kr separation. Previous research has shown NaX zeolite to be a selective adsorbent for xenon over krypton with a selectivity of about 6 with krypton concentrations ranging from 1 to 10,000 ppm.2 Jameson et al.3 showed that NaA zeolite had a selectivity of approximately 4 for binary mixtures of xenon and krypton at 300 K between 1 and 10 bar. They also used molecular simulations to show that ideal adsorbed solution theory (IAST) could accurately predict the selectivities and mixture behavior from the single-component isotherms.
Metal-organic frameworks,4-6 or MOFs, are a new class of nanoporous materials. Composed of organic linkers and metal corners, these materials self-assemble in solution to form stable, crystalline frameworks. Coordination bonds between oxygen and nitrogen atoms with metal centers allow for a variety of topologies, and choice of the organic linker allows one to tailor pore sizes and environments for particular applications. As a result, these materials have garnered much attention for hydrogen storage,7-9 separations,10,11 and catalysis.12-14 
A number of groups have investigated MOFs for separation of other gases. For example, Bae et al.15 used both experiments and simulation to show a mixed-ligand MOF effectively separates carbon dioxide from methane. Bae et al.16 also showed that exchanging fluorinated-methylpyridine into a MOF could substantially increase the selectivity of carbon dioxide over nitrogen due to the increased polar environment. Pan et al.17 synthesized a microporous MOF with 1D hydrophobic microchannels and demonstrated its ability to separate n-butane from other n-alkanes and olefins. Hartmann et al.18 showed that isobutene can be separated from isobutane using HKUST-1 in a breakthrough system. Yang et al.19,20 used molecular simulations to predict that HKUST-1 is a promising candidate for separation of carbon dioxide from both air and methane/hydrogen mixtures.
To date, there are a few publications that report the investigation of Xe/Kr separation using MOFs. Mueller et al.21 measured noble gas adsorption in IRMOF-1 and noticed significantly higher adsorption for the heavier gases, namely xenon and krypton, in MOF-filled containers relative to containers without MOF material. Building on these results, they built a breakthrough system filled with HKUST-1 and showed that a 94/6 molar mixture of krypton/xenon could be purified to over 99% krypton and less than 50 ppm xenon. Greathouse et al.22 recently simulated noble gas adsorption in IRMOF-1. They predicted that IRMOF-1 has a selectivity of about 2.5-3 for Xe over Kr at 298 K and pressures of both 1 and 10 bar.